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Prior Herbicide Use--Not Irrigation--is Critical to Herbicide
Efficacy / March 17, 2010 / News from the USDA Agricultural Research
Service

ARS plant
physiologist Dale Shaner has found that, while the herbicide atrazine breaks
down more quickly in fields that have been previously treated with the
herbicide, the amount of irrigation does not affect the degradation rate at
all. Click the image for more information about it.

Prior Herbicide UseNot Irrigationis
Critical to Herbicide Efficacy

Crop and herbicide use history are more critical to herbicide efficacy
and environmental safety than the timing and amount of irrigation water used,
according to Agricultural
Research Service (ARS) scientists.

In collaboration with CSU colleague Neil Hansen, Shaner and Wiles
compared the behavior of the herbicide atrazine in conventionally tilled corn
grown continuously year after year versus corn grown in three different crop
rotations. They tested various levels of tillage and irrigation, including no
irrigation.

The amount of irrigation usedincluding a total absence of
irrigationhad no impact on the rate of degradation of atrazine by soil
microbes in the top foot of soil. The only factors that made a difference were
prior herbicide use and the choice of crop sequences, with prior herbicide use
the most important factor by far.

Earlier studies, including one by Shaner, have shown that previous
applications of atrazine can predispose soil to more quickly degrade later
applications of the herbicide. But until now, it was not clear if other factors
such as cropping history and quantity of irrigation played a role.

There are two consequences of the more rapid dissipation of atrazine
in the plots with a history of use. The first consequence is a loss in weed
control. In the plots with the most rapid dissipation, weeds began to re-infest
the plots within four weeks after treatment, while the plots with the slowest
rate of dissipation remained weed-free through the growing season.

The second consequence is that atrazine leached more deeply in the
soil in the plots where it did not dissipate rapidly, but the herbicide did not
move below the top three inches of the soil in the plots where it was degraded
rapidly.